In Diablo terms, "synergies" are skills that have no (or very few) useful properties of their own, and exist only to boost the effect of some other skill. None of the skills in Diablo III are passives or synergies; Traits serve that function.
Synergies were common in Diablo II, in two forms.
- Passive skills, often called "masteries" in Diablo II, synergize benefits to other skills while having no effect themselves. One example is the Sorceress' Fire Mastery skill, which increased the damage of all of her fire skills.
- Synergies were skills that had their own function, but which were more useful (especially long term) for the percentage boosts they granted to other, more useful skills. Patch v1.10 revised the skill trees, adding numerous synergies of this type.
The Diablo III team has often spoken of their desire to make all the skills active and useful, rather than making any passive or synergistic. Traits handle the game's passive skill bonuses, and at least at the game's launch, there are unlikely to be any synergy skills.
@Diablo spoke about Diablo III's skill style in April 2011.
Will there be Synergys in Diablo 3 as there were in Diablo 2? IE, putting points in Firebolt to make Fireball do more damage. --NstnctGaming
Synergies were added in Diablo II (in 1.10) to spread out point spending and curb point hoarding. But had their own problems. The skill trees in Diablo III are far more open, you don’t have to spend down a linear tree. And passives are their own system. Couple that with runestones, and we don’t think there will be any need in Diablo III to bribe people with synergies. Traits are by their nature passive synergies in many ways, but no, skills do not ‘synergize’ with one another directly. --Diablo